1. Field of the Invention
The present Invention relates to an anisotropic stress buffer and a semiconductor device using the same.
2. Description of the Related Art
It is usual to interpose a soft body having a low elastic modulus as a stress buffer between two members, which are made of a material having a different thermal expansion coefficient from that of the other, when the two members are bonded together or when one of the two members is heated after being bonded, for the purpose of releasing a stress generated due to the difference in thermal expansion coefficient between the two members.
For example, when a semiconductor device is mounted to a substrate, the above-mentioned soft body having a low elastic modulus (stress buffer) is interposed between the two to facilitate the electric connection between circuits on the semiconductor device and the substrate. As a typical example, there may be a case wherein the stress generated due to the thermal expansion is absorbed by the stress buffer provided on a chip scale semiconductor device, which formed in a size generally equal to that of a semiconductor chip, is connected (surface-mounted) to the substrate. To enable the high density mounting, such a chip size semiconductor device is closer in size to a bare chip as compared to the usual semiconductor device. However, the thermal expansion stress due to the heat generation from the semiconductor chip itself is liable to be directly applied thereto. It means that it is indispensable to provide a stress buffer for releasing the stress in the horizontal direction.
FIG. 6 is a sectional view illustrating the above-mentioned conventional chip size semiconductor device. An insulating member 20 is mounted on a surface 10a of a semiconductor chip 10, on which an electrode terminal 12 of the semiconductor chip 10 is formed, without covering the electrode terminal 12. The insulating member 20 composed of an elastomer 22 forming a stress buffer, as a main layer, is laminated to the semiconductor chip 10 via an adhesive 24. A circuit pattern 30 has a land 32 to be connected to an external connection terminal at one end and a lead 34 which is a conductor layer at the other end. This circuit pattern 30 is supported by an insulating film 36 and forms together therewith a tape substrate 38. An adhesive 35 adheres the circuit pattern 30 to the insulating film 36. The tape substrate 38 is adhered onto the insulating member 20 at the land 32 and connected to the electrode terminal 12 by a bonding at the lead 34. A resin shield 90 shields exposed portions of the lead 34 and the electrode terminal 12 after the lead 34 has been bonded. The external connection terminal is, for example, solder balls 40 connected to the land 32.
The stress buffer is useful for improving the reliability in electric connection not only when the above-mentioned chip size semiconductor device is mounted onto the substrate but also when a usual semiconductor device, in which a semiconductor chip is mounted to a semiconductor package, is mounted to a substrate.
As described above, to solve problems of stress caused by the difference in thermal expansion coefficient when two members are bonded together, a stress buffer made of a soft material having a small elastic modulus (a low elastic modulus member) may be used for releasing the stress. However, there is a still further problem in that the elastic modulus of such a stress buffer is too small to result in a proper balance between the softness and rigidity in all the directions.
Also, there may be a case wherein a low elastic modulus (softness) is required in one direction, while a high elastic modulus (rigidity) is required in the other direction. For example, in the field of the above semiconductor device, the stress buffer having a low elastic modulus in a planar direction in parallel to a mounting surface of the substrate and a predetermined rigidity (Young's modulus higher than a predetermined value) in a direction vertical to the mounting surface. If the stress buffer has the predetermined rigidity in the thickness direction thereof, it is possible to electrically connect the semiconductor chip with the semiconductor device via a wire bonding. Since the prior art stress buffer has a small elastic modulus in all the directions, it is difficult to carry out the wire bonding on the circuits formed on the surface of the stress buffer. This is because, while the wire bonding must be carried out under a predetermined load while applying heat and ultrasonic wave, the stress buffer operates as a cushion to cancel the load.
In this regard, the electric connection according to the wire bonding has a high reliability backed up by actual results. Also, the wire bonding is capable of preventing the quality of the product from deteriorating and the production cost from rising in comparison with other methods. Further, since the connection structure can be simplified, it is possible to meet a demand for the high density (micro) mounting or the shortening of a production process.